This is an open-label (a medical research study in which participants and researchers are told which treatments the participants are receiving, "unblinded"), single-arm, prospective study (study following participants forward in time) of TTS-fentanyl matrix form in knee osteoarthritis participants. The study consists of 3 phases: a screening phase, an open label treatment phase consisting of 2 periods, and an evaluation phase. The first patch will be applied on the first day of treatment phase by the investigator, and sufficient patches until Day 30 will be provided to the participant with the instructions to apply the patch. The TTS-fentanyl dose will normally be increased, if needed by 12.5 microgram per hour taking into account the daily dose of supplemental paracetamol required by the participant. Efficacy with regard to pain control will be recorded principally by the participant via questionnaires in a daily diary. This record will be used to support more detailed assessments at study visits on Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire. Participants' safety will be monitored.